Traditional touch screen displays are formed by combining a display that is typically formed on a first substrate containing an array of thin film transistors with a touch screen that is formed on or in contact with a second substrate. Typically this second substrate is then positioned between the display substrate and the user.
This construction creates a number of problems that should be overcome. First, because the display and the touchscreen are created from separate materials and assembled together, displays with integrated touch screens can be relatively expensive. To decrease this cost, the touch screen is often formed using low cost electronics, which decreases the sensitivity, response time, or selectivity of the touch screen. One of the most common approaches is to apply a passive matrix addressing approach in which rows of sensors within the touch screen are addressed at any one time or an approach in which signals are read only from rows and columns of electrodes, without exact two-dimensional isolation. For example, Wong et al. in US Publication No. 2008/0158171, entitled “Digitizer for flexible display” discusses a flexible display layer using passive addressing as depicted in FIG. 6 of that publication. The use of active addressing techniques, using, for example, arrays of thin film transistors (TFTs) and capacitors to permit a signal to be accumulated and read out upon demand is more desirable as it can provide better user responsiveness and sensitivity but can be very expensive.
Another issue is that the touch screen is often not fully transparent, often including reflective metal traces or other non-transparent elements to permit electrical signals to be captured and conveyed to a processor external to the touch screen. Because this touch screen is often placed over top of the display and is not transparent, it typically reduces the contrast and therefore, the perceived quality of images presented on the underlying display. Further, the touch screen is often not in optical contact with the display and therefore, light emitted by the display can be reflected between the two substrates, further reducing the effective contrast of the display. As an example, Cheng in US Publication No. 2008/0180399, entitled “Flexible multi-touch screen” discusses a flexible display having a “transparent panel that is positioned in front of the flexible display device”. However, in discussing the “transparent panel” in detail this embodiment indicates that the gap between sensors is preferably made small to increase the sensing area and to reduce optical differences between the space and the transparent sensors. Therefore, even in this touch screen, which is called “transparent,” Cheng acknowledges that the sensors have optical properties that vary from the optical properties from other regions within the overlay and are thus not fully transparent.
Another issue is that because the touch screens do not always have the ability to discretely sample data from two-dimensional locations, it is often difficult to determine where the display is being touched when the user touches the display in more than one location. For example, Roberts in U.S. Pat. No. 7,196,694, entitled “Force sensors and touch panels using the same” and Laitinen et al. in US Publication No. 2007/0103449, entitled “Cost efficient element for combined piezo sensor and actuator in robust and small touch screen realization and method for operation thereof” discuss touch sensitive screens in which piezo actuators for measuring stress or strain are placed at the corners or edges of a substrate. However, because there are only four sensors and a relatively rigid surface is constrained only at the location of the sensors, it is practically impossible to distinguish a force applied by two fingers at two distinct locations from a single force applied midway between the two distinct locations using a touch panel of this type.
An additional problem with these touch screens is the parallax that is induced due to the fact that the touch sensor has a finite thickness and is arranged in front of the image plane. For this reason, the user's perceived touch location can be affected by his or her head position with respect to the center of the display. This further complicates usage of the display.
It is also known to use other user input or interaction besides touch to improve the interaction between the user and a display. In one example, it is known to incorporate bend sensors, for example, sensors for measuring strain in a substrate, to determine the degree to which a flexible display is bent and to enable the display to be updated as the display is bent. For example, Narayanaswami et al in US Publication No. 2006/0238494, entitled “Flexible displays as an input device” discuss incorporating bend sensors into a flexible substrate to permit the display to determine the degree to which a user bends the display. This publication teaches that the output from the bend sensors can be combined with other sensor values, such as those obtained from a touch sensor, to provide a rich user interaction. Such an interaction paradigm is interesting but does not provide for the direct manipulation of objects that is provided by a touch screen.
There is a need for a flexible display with a touch screen that does not overlay the image that is created to avoid degrading the perceived quality of the image, is useful with a flexible display, does not exhibit parallax, has improved sensitivity and enables multi-touch interfaces. There is also a continuing need for a flexible display which provides information in addition to touch location, such as the force or the rate at which the user presses the display.